1. Field of the Invention
This invention pertains generally to optical motion detectors used in alarm systems, and more particularly to a passive infrared (PIR) intruder detection system capable of ignoring pet intrusions without compromising human intrusion detection sensitivity.
2. Description of Related Art
Security systems typically utilize infrared motion sensing devices, such as passive infrared detectors, for registering the presence of unauthorized personnel intruding within a protected region. Optics, typically comprising an array of lens elements or an array of segmented mirrors, focus infrared energy from the protected region onto a sensor. A typical sensor contains two infrared sensing elements. An array of segmented mirrors or an array of lens elements is used to produce multiple fields of view, also known as detection zones. The geometric relationship between a sensing element and a lens element defines a detection zone, a volume which emanates from the detector.
The geometry of each detection zone can be defined by projecting each sensing element through the optical center of each lens element and bounded as the detection zone intersects the floor or attains a length equal to the manufacturers advertised range. The array of resulting detection zones defines the detection pattern. The protected region is the envelope of the detection pattern as truncated by the walls of the room containing the detector. Registering a sufficient level of infrared energy or change in infrared energy with respect to time (motion) in any detection zone will cause the system to generate an intrusion signal. One common type of sensor is a pyroelectric infrared sensor designed for registering a change in radiation intensity within the infrared spectrum.
A number of security applications exist where the conventional detection field geometry is not suitable to the application. One problem encountered in security applications is discriminating between human intruders and the motion of pets or other small animals (i.e. dogs or cats) moving in, or through, the protected region. A number of intrusion detectors have incorporated features in an attempt to discern between these two forms of thermally active objects. One problem with many of these detectors is that they attempt to discriminate between human and animal intrusions based on the intensity of the infrared (heat) signature. To prevent false alarms caused by pets, these intrusion detector mechanisms generally require reducing their sensitivity by a significant factor.
It has been found in testing, however, that a human heat signature in some cases can have a lower intensity than that of a small animal, such as a dog. Many average size dogs, especially those having a high thermal signature such as short-haired dogs and those with a high metabolism, will exceed the infrared threshold. Increasing the infrared threshold level is not a valid option as it was also found that an individual with a low thermal signature, such as a person of small stature, wearing thick clothing, or having a large amount of body fat, could provide a thermal signature less than that of many dogs, wherein the human intruder would go undetected.
Consequently, these intensity-based approaches to discerning human intrusion from animal motions are still subject to generating false alarms, while the sensitivity reductions intended to prevent false alarms from small pets can allow human intruders with a small thermal signature to pass undetected.
One example of an existing system utilizes vertically-stacked lens elements within a lens array to enlarge the vertical image zone in an attempt to collect more of the infrared radiation of an upright object (human) versus a “shorter” dog. It is presumed that signals being received from a pet such as a dog will not cross the signal threshold, while the infrared radiation intensity from a human is expected to exceed the threshold and generate an output signal.
Another technique registers intrusion in response to information collected as an object traverses multiple zones of a multiple element pyroelectric infrared sensor grid. This method requires a significant distance to operate and is not amenable for use when the object moves directly toward the sensor.
One technique for discriminating humans from pets is to increase the sensitivity of the upper detection zones. This is accomplished by increasing the area of the upper lens elements. This approach assumes that pets will not be able to enter these upper detection zones. Since these detection zones are angled below horizontal, they will detect a pet at a distance. If the room is of sufficient size these detection zones can drop down to the height of the pet making the detector subject to false triggering by pets. Since the short range detection zones are made less sensitive to preclude an alarm caused by a pet, these detectors are subject to non-detections of human intruders, such as those that are heavily clothed, or that otherwise present a small heat signature.
Attempts have also been made to combine other forms of sensors with the PIR detector to discern pet motions from human intrusions. One example is an active microwave system utilized in combination with a PIR detector. This form of intrusion detection system requires complex signal processing on the inputs to evaluate the results. Initial costs as well as set-up costs are high for such an approach.
Another form of common approach for distinguishing pet intrusion from human intrusion is that of utilizing multiple sensors with multiple lenses within a single housing. In one example, a first pyroelectric infrared sensor and lens array is mounted at shoulder height with detection zones receiving infrared energy over a substantially horizontally oriented detection band. A second pyroelectric infrared sensor and lens array is retained below the first sensor and lens array producing a second horizontal band of detection zones located directly below the first band. Activity on both sensors is required to trigger an alarm, presuming again a standing individual.
A disadvantage of mounting a sensor at shoulder height is that it can be easily damaged or blocked. In similar approaches using two sensors and two lens arrays, a series of horizontal bands of detection zones is produced in which the bands produced by one sensor/lens arrangement is interlaced vertically with those of the other sensor/lens arrangement. Since prior art which utilizes this technique uses the same focal length for all lens elements, the upper bands of detection zones for each sensor/lens arrangement overlap and create a false alarm issue for long range detection. Consequently, this technique is similarly prone to false triggering as prior attempts at a solution.
FIG. 1 illustrates a conventional dual-element pyroelectric infrared sensor having one positive and one negative sensed element. These sensors may be utilized in a conventional intrusion detection device which is configured for detecting infrared energy changes which arise in the field of view of the detection device.
FIG. 2 illustrates a twenty four (24) element lens array shown with the lens elements arranged to produce four tiers of lens elements. When this lens array is combined with a two element pyroelectric infrared sensor shown in figure one, 48 detection zones are produced, one positive and one negative zone for each lens element. The tiers of lenses are configured in an attempt to discriminate humans from pets, however, the intensity of infrared radiation from a short-haired animal, or one with a high metabolic rate, or other similar situation can approximate or exceed that of a human. Furthermore, a human intrusion may not be registered by such a sensor if the person is short, overweight, has a low metabolism, is wearing thick clothing or otherwise exhibits a low infrared radiation signature. In many designs, the horizontal bands of detection zones overlap one another in the vertical direction.
FIG. 3 and FIG. 4 show a comparison of human detection (FIG. 3) in relation to detection of a dog (FIG. 4) with regard to how the detection zones produced by the lens of FIG. 2 detects an intruder. The conventional system anticipates that infrared energy from a tall object will be received over more tiers of lenses thereby increasing the sum of infrared energy that is focused on the sensing elements and the chance of crossing the infrared detection threshold. The lenses corresponding to the diagram are marked in FIG. 2 as lenses A, B, C, D. It will be noted however, that a significant infrared source can trigger an intrusion alert regardless of whether it occurs in one or more vertically aligned zones.
In the above techniques, when pet-immune detection is selected, a compromise is required between sufficient sensitivity for detecting human intruders and the necessity of ignoring the motion of pets. These methods rely on the use of interchangeable sensors or lenses for desensitizing the security system for use in an environment that may include small animals, such as pets. This requires additional cost, setup expense, experienced installation personnel, and can lead to additional overall maintenance costs.
Therefore, a need exists for an intrusion detection apparatus that is able to reliably ignore the motions of small animals while accurately detecting intrusions by humans, and which may be configured for pet and non-pet detect situations without the need to change hardware. The present invention satisfies those needs, as well as others, and overcomes deficiencies in previously developed solutions.